Seema can sell 10 sweaters for $70 each, 20 sweaters for $60 each, 30 sweaters for $50 each, 40 sweaters for $40 each, and 50 sweaters for $30 each. Her marginal cost of production is constant at $30 for each additional unit (or sweater) produced. If she behaves like a perfectly competitive producer, what is the number of sweaters she will sell?

A. 20

B. 30

C. 40

D. 50